1. Technical Field
The present invention relates to transistor switching circuits; and more particularly, to a power switching circuit utilizing a plurality of parallel connected switching transistors.
2. BACKGROUND ART
The advancement in power electronics depends upon the power switch device technology. Because of transistor switching limitations, it is sometimes necessary, in high power applications, to parallel the switching transistors in order to obtain the desired current carrying capability. When paralleling a plurality of transistor switches, mismatches in the characteristics of individual transistors can cause the devices to share the current improperly. This results in one or more of the transistors being overloaded, which, in turn, causes failure of the switching circuit. In order to prevent one of the transistor switches from carrying more than it's share of the total current, it is well known to use ballasting resistors in series with each of the parallel connected transistors to force current sharing. The series connected ballast resistors provide for sharing the current in the switch on-state, but do not force the sharing during the transition period between the on and off conditions of the switch. If one transistor turns on faster than the other, and conversely if one transistor turns off slower than the other, then the switching device can be destroyed due to inordinate transition current slugs.
Although it is recognized that successful paralleling can be achieved due to the use of advanced packaging, careful layout, homogeneous dice and good thermal management without the use of ballast resistors, it is difficult to prove that current sharing will occur well enough to guarantee that the devices will always operate within specified limits.
In light of the foregoing, there is a need for a power transistor switching device where parallel connected transistors share the current equally not only during the on o state of the parallel connected transistors, but also during the transition between the on and off states of the switching devices.